1. Field of the Invention
The present invention relates generally to steam turbine blades and, more particularly, to an L-3R tapered twisted integral shroud rotating blade having improved performance characteristics.
2. Description of the Related Art
Rotating and stationary blades of a steam turbine are arranged in a plurality of rows or stages. The rotating blades of a given row are usually shaped identical to each other, except in the case of mixed tuned blades, and are mounted in corresponding mounting grooves provided in the turbine rotor. Stationary blades, on the other hand, are mounted on a cylinder which surrounds the rotor.
The rotating blades of a turbine, regardless of which row they are in, typically share the same basic components, as shown in FIG. 1 herein. Each has a root portion 13 receivable in the corresponding mounting groove of the rotor, a platform portion 15 which overlies the outer surface of the rotor at the upper terminus of the root 13, and an airfoil portion 17 which extends upwardly from the platform portion.
Stationary blades also have airfoils, except that the airfoil portions of the stationary blades extend downwardly towards the rotor. The airfoil portions of both stationary and rotating blades typically include a leading edge 22, a trailing edge 26, a concave pressure side surface 18, and a convex suction-side surface 14.
The airfoil shape common to a particular row of blades differs from the airfoil shape for every other row within a particular turbine. In general, no two turbines of different designs share airfoils of the same shape.
The structural differences in airfoil shape result in significant variations in aerodynamic characteristics, stress patterns, operating temperature, and natural frequency of the blade. These variations, in turn, determine the operating life of the turbine blade within the boundary conditions (turbine inlet temperature, pressure ratio, and rotational speed), which are generally determined prior to airfoil shape development.
Development of a turbine for a new commercial power generation steam turbine may require several years to complete. When designing rotating blades for a new steam turbine, a profile developer is given a certain flow field with which to work. The flow field determines the inlet angles (for steam passing between adjacent blades of a row), gauging, and the force applied on each blade, among other things. "Gauging" is the ratio of throat to pitch, "throat" is the straight line distance between the trailing edge of one blade and the suction surface of an adjacent blade, and "pitch" is the distance in the tangential direction between the trailing edges of the adjacent blades.
These flow field parameters are dependent on a number of factors, including the length of the blades of a particular row. The length of the blades is established early in the design of the steam turbine and is essentially a function of the overall power output of the steam turbine and the power output for that particular stage.
Referring to FIG. 2, two adjacent blades of a row are illustrated in sectional views to demonstrate some of the features of a typical blade. The two blades are referred to by the numerals 10 and 12. The blades have convex, suction-side surfaces 14 and 16, concave pressure-side surfaces 18 and 20, leading edges 22 and 24, and trailing edges 26 and 28, respectively.
The throat is indicated in FIG. 1 by the letter "O", which is the shortest straight line distance between the trailing edge of blade 10 and the suction-side surface of blade 12. The pitch is indicated by the letter "S", which represents the straight line distance between the trailing edges of the two adjacent blades.
The width of the blade is indicated by the distance W.sub.m, while the blade inlet flow angle is .alpha.1, and the outlet flow angle is .alpha.2.
".beta." is the leading edge included flow angle, and the letter "s" refers to the stagger angle.
When working with the flow field of a particular turbine, it is important to consider the interaction of adjacent rows of blades. The preceding row affects the following row by potentially creating a mass flow rate near the base which cannot pass through the following row. Thus, it is important to design a blade with proper flow distribution up and down the blade length.
The pressure distribution along the concave and convex surfaces of the blade can result in secondary flow which results in blading inefficiency. These secondary flow losses result from differences in steam velocity between the suction and the pressure surfaces of the blades.
A rotating blade can be "free-standing", in that there is no interconnection between adjacent blades in the upper region of the airfoils, or it can be interconnected at the tip with an adjacent blade or blades through a shroud segment. Shroud segments can be either integrally formed on the tip of each blade, or separately connected by attachment to a tenon formed on each blade tip.
Moreover, rotating and stationary blades can be either straight parallel-sided or tapered twisted. In a tapered twisted blade, center lines of the leading and trailing edges are non-parallel, owing to the changing geometry of the blade along its length. Conversely, since each cross section of a parallel-sided blade is identical, the center lines of the leading and trailing edges will be parallel.
The fourth stage of a Westinghouse Electric Corporation (the Assignee of the present invention) building block (BB) 71 low pressure turbine presently includes a row of rotating blades of the aforementioned parallel-sided configuration. This blade was designed without regard to three-dimensional flow field analysis.
Tuning of resonant frequencies is an additional important consideration when undertaking the design of a new blade. In some instances, different blade materials will be chosen depending on design criteria. The particular material used has a direct effect on Young's modulus, which in turn has an effect on blade frequency. Thus, according to currently available blade technology, a blade design having tuned frequencies with one material may have untuned frequencies when another material is substituted (for example, where the nickel percentage is different in one stainless steel than another).